1. Field of Invention
The present invention relates to vertical and horizontal scanning correction systems for video displays, and in particular, to vertical and horizontal scanning correction systems for large video displays with flat screens.
2. Description of the Related Art
As the display areas, or screens, of video displays have become larger and flatter, maintaining geometrically correct, e.g., linear, vertical and horizontal raster dimensions has become more problematic. This is particularly true for shallower cathode ray tubes (CRTs) due to the greater angles (vertical and horizontal) of deflection of the electron beams which create the raster. As is well known in the art, the reason for distortions in the larger and flatter screens is that the comers of such screens are further from the point of deflection of the electron beams as compared with the center of the screen. Thus, the electron beams are moved along greater horizontal and vertical dimensions at the extreme angles of deflection, i.e., the corners.
Referring to FIGS. 1A-1D, examples of some common raster distortions include: east-west pincushion (FIG. 1A); trapezoidal (FIG. 1B); east-west pincushion unbalance, or bow (FIG. 1C); and asymmetrical trapezoidal, or parallelogram (FIG. 1D). Pincushion distortion results primarily from the fact that in a cathode ray tube (CRT) the traveling distance of the electron beam from the electron gun to the picture tube screen is longer at the four corners than for the center region of the screen. Hence, for a given angular change in deflection, the electron beam is moved a greater distance across the screen at the comers than in the center region, as represented in FIG. 2.
Other asymmetries are often introduced due to the quality and/or alignment of the deflection yoke on the neck of the CRT. Referring to FIGS. 3A and 3B, distortion along the vertical dimension is also common. While north-south pincushion distortion is generally corrected by a self-converging deflection yoke (designed for converging the three electron beams from the R,G and B electron guns to the same color dot triplet) vertical deflection is nonetheless often distorted due to the aforementioned increase in electron beam deflection at the outer regions of the display. However, such vertical deflection distortion, the effect of which is image elongation near the top and bottom edges of the raster, can be corrected by introducing some "S" shaping to the otherwise linear sawtooth signal which controls the vertical deflection. Hence, as shown in FIG. 4, by shaping the ramp portion of the sawtooth vertical deflection control signal to resemble an "S" rather than a straight line, the distorted raster of FIG. 3A can be corrected to appear as the raster shown in FIG. 3B.
Referring to FIG. 5, similar correction of east-west distortions can be made by amplitude modulating the envelope of the sawtooth signal which controls horizontal deflection, as well as adding a signal component at the vertical field frequency for correcting east-west unbalance distortions.
Conventional vertical and horizontal scanning control systems used to correct such vertical and horizontal distortions, have, to various degrees, suffered from a number of problems. For example, the signal used to generate the "S-curve" shaping for the vertical linearity correction should be a waveform having a parabolic shape, or an approximation thereof. One technique for achieving such a shape has been to approximate a parabolic curve by performing a full-wave rectification of the sawtooth vertical scanning signal. However, full-wave rectification is not perfectly symmetrical. While the typical symmetry of the rectified signal may be adequate for smaller displays, larger displays, particularly those with flatter screens, require much greater precision and symmetry to avoid non-linear scan lines at the outermost areas of the raster. Furthermore, for displays having flat screens, the second order nature of a parabolic signal is insufficient for providing adequate correction at the very top and bottom of the raster.
Another problem experienced by conventional displays involves horizontal correction for very large displays having flat screens. As noted above for the case of vertical correction, the second order nature of a parabolic waveform is insufficient for providing adequate horizontal correction for large displays with flat screens. Additionally, even if adequate vertical and horizontal correction can be achieved, adjustment of the vertical positioning of the raster within the display area causes the vertical center of the raster to change, thereby affecting the position at which maximum horizontal correction is applied, and thereby making such correction non-symmetrical along the sides of the raster.
Accordingly, it would be desirable to have a vertical and horizontal scanning control system for large video displays with flat screens which avoids or otherwise compensates for the foregoing problems associated with present conventional systems.